The present invention concerns an adaptable rate pacemaker method and apparatus in which the ventilatory (or anaerobic) threshold break point is determined without the need for a maximal exercise test.
Implanted cardiac pacemakers are employed to assist patients suffering from electrical conduction disorders of the heart. Such pacemakers originally restored a normal heart rate by providing a single fixed rate of paces, or a narrow range of externally programmable pacing rates. These early pacemakers failed to meet patients"" metabolic demands during exercise. Consequently, so-called xe2x80x9crate adaptivexe2x80x9d or xe2x80x9crate responsivexe2x80x9d pacemakers have been developed. These pacemakers sense any of a variety of different parameters that are correlated to physiologic need and adjust the pacing rate of the pacemaker in response thereto. See generally U.S. Pat. No. 5,578,064 to Prutchi. Rate adaptive pacemakers require (a) an input parameter which can be monitored as an indication of the metabolic need of the patient, and (b) a program or model by which the pacing rate delivered to the patient by the pacemaker at a particular time is determined from the input parameter.
In normal subjects, pulse is commonly monitored as an indication of a subject""s metabolic need during work or exercise. Of course, in patients requiring a pacemaker, a pulse is not available as an input parameter because the pulse is driven by the pacing rate of the pacemaker itself. Accordingly, common input parameters for adaptive rate pacemakers are stroke volume of the heart and the minute volume of respiration, both of which can be inferred from impedance measurements. See, e.g., U.S. Pat. No. 5,578,064 to Prutchi, U.S. Pat. No. 5,201,808 to Steinhaus et al.; and U.S. Pat. No. 4,901,725 to Nappholz et al. The problem of establishing a model to determine the pacing rate to be given in response to a particular input parameter is a separate issue.
T. Lewalter et al., PACE 18, 1374 (1995), describes a low intensity treadmill exercise (LITE) protocol for rate adaptive programming of minute ventilation controlled pacemakers. The purpose of the study was to determine the physiological relationship between heart rate and minute ventilation (HR/VE) during peak exercise testing, to develop a database for appropriate rate adaptive slope programming of minute ventilation controlled pacemakers. As an alternative to peak exercise testing, the LITE protocol was used. The LITE protocol was performed on a treadmill with the collection of breath-by-breath gas exchange, and linear regression analysis used to determine the HR/VE slope below and above the anaerobic threshold and during the early, dynamic phase of low intensity exercise with the LITE and the ramped incremental treadmill exercise (RITE). It was determined that the LITE protocol can be performed as a substitute for peak exercise stress tests to determine the correct pacemaker rate response factor in order to obtain a physiological heart rate to minute ventilation relationship for the appropriate matching of paced heart rate with patient effort. A problem with this procedure is, however, the need for extrinsic minute volume measurement with an external flow sensor. This requires that the procedure be performed in a clinical setting with a treadmill or the like. Thus, the patient is unable to choose the particular exercise on which programming of the pacemaker will be based. It would be preferable to provide a procedure for accurately determining the ventilatory threshold breakpoint for adaptive rate pacing that can be entirely incorporated into an implantable patient device. In this manner, the patient would be able to choose an exercise of any modality, and perform submaximal exercise, and then the device would automatically determine the appropriate adaptive rate pacing response factor.
A method for automatically determining the ventilatory (or anaerobic) threshold breakpoint for adaptive rate pacing without the need for directly measuring anaerobic threshold or ventilatory threshold in a human subject carrying an implanted pacemaker, is disclosed. The method comprises: (a) positioning a first sensing electrode in the heart or superior vena cava of the patient, the first sensing electrode connected to the implanted pacemaker; (b) positioning a second sensing electrode in the thoracic region of the patient and spaced apart from the first sensing electrode; (c) determining the chest wall impedance of the patient between the first sensing electrode and the second sensing electrode; then (d) measuring the ventilation (e.g., the minute ventilation) of the subject from the chest wall impedance during submaximal exercise by the patient; and then (e) determining the ventilatory threshold breakpoint of the patient from the measured ventilation.
The second sensing electrode is typically positioned outside the pleural cavity of the patient, and is preferably positioned on the surface of the implanted pacemaker. A preferred embodiment of the invention further includes the step of measuring respiratory rate (RR) concurrently with the step of measuring the ventilation of the subject. The determining step then comprises determining the ventilatory threshold breakpoint of the patient from both the measured ventilation during submaximal exercise and the measured respiratory rate during submaximal exercise. Preferably the method is implemented by calculating a parameter (e.g., by regression analysis) from the measured ventilation, with the parameter being respiratory rate change during submaximal exercise, tidal volume change during submaximal exercise, or tidal volume during steady state submaximal exercise, and then determining the ventilatory threshold breakpoint from at least one of those calculated parameters. Peak ventilation can be determined in like manner.
One preferred embodiment of the foregoing method further includes the steps of: (i) determining the onset slope of tidal volume (Vt) during the submaximal exercise; (ii) determining the onset slope of respiratory rate (RR) during the submaximal exercise; (iii) determining the steady state of Vt during the submaximal exercise; (iv) determining the steady state of RR during the submaximal exercise; and (v) determining minute ventilation (VE) during a rest period prior to or following the submaximal exercise. The determining step then comprises determining the ventilatory threshold breakpoint of the patient from all of the measured ventilation during submaximal exercise; the measured respiratory rate; the onset slope of tidal volume during the submaximal exercise; the onset slope of respiratory rate, the steady state of Vt during submaximal exercise; and the steady state of RR during the submaximal exercise; VE during the submaximal exercise; and VE during the rest period.
The measuring step may be initiated under operator control, which operator may be the patient, a medical professional, or other. Once the ventilatory threshold breakpoint is determined, the pacemaker may then be programmed to reduce the rate of increase in the pacing rate (which is typically increased in rate in response to detection of increased ventilation by the chest wall impedance sensor) at the determined ventilatory threshold breakpoint. The programming step is preferably carried out automatically, without further operator intervention. Further, the pacemaker is preferably programmed so that the maximum pacing rate is matched to and delivered upon attaining the peak ventilation, which peak ventilation is determined as described above.
A second aspect of the present invention is a rate-responsive cardiac pacemaker. The pacemaker comprises a pacing circuit configured for pacing a patient""s heart as a function of a pacing signal at a pacing rate up to a maximum pacing rate; a sensing circuit configured to sense ventilation by said patient; and a control circuit operatively associated with said sensing circuit and configured (via hardware, software, or combinations thereof, implementing the methods described above) to determine a ventilatory threshold break point from the sensed ventilation. In use, the pacing signal is modified by sensed ventilation (and optionally other signals). The control circuit is configured to modify the pacing rate in response to the ventilatory threshold break point determined thereby. In addition, the control circuit is preferably configured to determine a peak ventilation in response to the sensed ventilation, and match that sensed ventilation to the maximum pacing rate of the device (i.e., so that the maximum pacing rate is delivered when the peak ventilation is detected).